This invention relates to filters; more particularly the present invention relates to filters for ink which is being fed to the printhead of a printer.
The final, or xe2x80x9clast ditchxe2x80x9d, filter currently used in ink jet printers comprises a disc filter, fabricated from stainless steel, retained within flexible tubing, typically fabricated from PTFE, through which flexible tubing and retained filter the printing ink is fed to the printhead. The primary function of this final filter is to protect the actuator of the printer from contamination by the ingress of dirt once the actuator has left its controlled manufacturing environment. It is known that the inadvertent ingress through the filter of one particle above 20 xcexcm in size would result in printhead failure. Such stringent filtration has hitherto required relatively massive filters which are deployed externally to the printhead cover.
The present invention seeks to provide an improved filter in which these problems are mitigated.
According, therefore, to one aspect of the present invention there is provided a filter assembly, preferably an ink jet print head filter assembly, for ink for a printer, which filter assembly comprises:
at least one pair of supported filter elements which are in stacked arrangement;
a filter housing; and
means contained within the filter housing for conveying the ink through the filter elements so that the ink flow is either into or out of the volume between the filter elements.
The filter assemblies of the present invention can have small size but a large filter area.
It is preferred that there is one pair of stacked filter elements; and that the filter elements are supported at least at their periphery.
In one embodiment, the conveying means comprises at least one manifold, contained within the filter housing, for conveying ink from a supply through the filter elements.
Preferably, the or each pair of filter elements is supported by a respective filter support formed from plastics material. The filter support and manifold may be a unitary structure.
It is particularly preferred that the or each filter support, and manifold, is formed by injection moulding, especially by reactive injection moulding.
By proceeding in this manner, it is found possible to eliminate flexible tubing which has been found to exhibit the disadvantage that particles can become dislodged therefrom on flexure of the tubing. Furthermore, injection moulded components have been found to have low shedding properties and can also have a high surface finish leaving little possibility for dirt to become entrapped.
The filter elements may comprise finely woven wire the mesh of which is effective to prevent passage of particles of at least 20 xcexcm in diameter. Preferably, the wire is of a metal which, in service, is resistant to corrosion by the fluids being filtered. Examples include stainless steel, titanium or gold with stainless steel being preferred. DUTCH TWILL weave has been found to be very suitable, especially 320xc3x972000 DUTCH TWILL (320 wires/inch (approximately 126 wires/cm) in one direction of weave, 2000 wires/inch (approximately 787 wires/cm) in the other direction). The filter elements may be suitably adhesively bonded to the filter support. Such woven wire media, especially stainless stee!, have been found to have very low shedding properties; furthermore, an adhesive bead seals cut edges of the woven wire to reduce further the possibility of shedding as well as to secure the filter element in position.
The filter elements may comprise plastics membranes, for example, PTFE (polytetrafluoroethylene) membranes.
In order to reduce further the likelihood of shedding, each pair of filter elements may comprise opposing faces of a filter. The use of a unitary, wraparound filter, instead of individual filter elements, also enables the use of adhesive sealing to be avoided.
The filter assembly of the present invention can be provided in very compact form (in particular, of a width less than that of the supplied nozzles of the printer) yet can, in service, suitably furnish a pressure drop across the filter housing of less than 10% of the pressure drop across the filter element. Preferably, the filter element has a contact area effective to ensure, in service, a pressure drop thereacross of less than 16 mm ink. In general, the filter assemblies of the present invention enable the pressure drop across the filter housing to be small relative to the pressure drop across the filter element while necessitating only a small internal ink volume. The filter housing is desirably tapered in the downstream direction to facilitate the expulsion of air from the filter housing.
This invention particularly provides an ink jet print head assembly of generally rectangular cross-section wherein:
the height of the assembly is the minor dimension and is effective to ensure, in service, a pressure drop across the filter assembly of less than 16 mm ink;
the width is less than the width of an array of ink jet nozzles supplied through the filter; and
the length of the filter housing corresponds substantially to the distance between the nozzle array and the electrical connectors to the drive circuitry for operating the print head.
Minimising the height of the filter assembly in this way allows it to fit easily beneath the print head cover and/or allows the print head cover to have a streamlined, low profile. Not only does this result in a product that is pleasing to the eye, such a configuration allows print heads to be stacked with their nozzle arrays parallel to one another with minimum separation.
The filter assembly of this invention also suitably additionally comprises interfacial means for integrating the filter in line with an ink supply and with a printer. In accordance with a further aspect of this invention, there is provided a printer, preferably an ink jet printer, which comprises a filter assembly in accordance with the herein described invention. In a particularly preferred embodiment of this aspect of the invention; the filter assembly is located beneath the printhead cover.
In a further aspect of this invention there is provided a method of filtering ink for a printer, which method comprises:
causing the ink to flow through at least one pair of supported filter elements which are in stacked arrangement, the flow being either into or out of the volume between the filter elements; and
supplying filtered ink to the printer.